Electron discharge device



Dec. 11, 1962 C. K. BIRDSALI.. ETAL ELECTRON DISCHARGE DEVICE Filed March 7. 1955 N QQ IN VEN TORS United States Patent Ofice 3,068,377 Patented Dec. 11, 1962 3,062,377 ELECTRON DISCHARGE DEVICE Charles K. Birtlsall, Los Angeies, and George R. Brewer,

Palos Verdes Estates, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Mar. 7 1955, Ser. No. 492,664 1 Claim. (Cl. 315-36) This invention relates to microwave tubes and more particularly to a high-gain traveling-wave tube amplifier. v As microwave tubes find application in new fields, it isfrequently desirable to obtain as much gain as possible from such tubes. A certain known but unavoidable input or electromagnetic excitation loss in a traveling-wave tube amplifier reduces gain. The loss takes place when the total input energy divides into three forward waves. .Only one of the waves exists as a growing Awave and hence vthe energy divided between the two other waves becomes a total loss.

It is therefore anv object of the invention to provide a relatively high-gain traveling-wave tube.

, It is another object of the invention to provide means whereby the radio-frequency input loss in a travelingwave tubeampliiiermay be reduced.

In accordance with the present invention, a relatively short input helix anda relatively long principal helix are maintained at different average direct-current potentials in a traveling-wave tube. The potential of the input helix is maintained at a value which will provide maximum gain at the output end of the principal helix but generally at a lower potential than that of the principal helix in order to minimize the radio-frequency input loss of the tube for the production of maximum gain. The principal helix is then maintained at a potential for optimum gain. Means are also provided to couple electromagnetic wave energy from the input helix to the principal helix.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawing-is for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.

' FIG. 1 is a sectional view of an embodiment of the traveling-wave tube amplifier of the invention shown with associated circuitry; and

Y FIG. 2 is a broken section of a traveling-wave tube in which an alternative embodiment of the invention is illustrated.

t Referring to the drawing, in FIG. l an embodiment of the microwave amplifier of the invention is shown comprising a traveling-wave tube 10 including an input matching cavity 12 having a coaxial input cable 14 connected thereto and an output matching cavity 16 connected to a coaxial output cable 18. An envelope 20, which provides the evacuated chamber of travelingwave tube 10, consists of a long cylindrical structure which has an enlarged portion at its left extremity as illustrated in FIG. 1.

Within the enlarged portion at the left extremity of the envelope 20, there is located an electron gun 22 for developing an electron stream. Gun 22 comprises a cathode 24 with a heater 26, a focusing electrode 2S and an accelerating anode 30. Heater 26 is connected across a source of potential, such as battery 32, the negative side of heater 26 being connected to cathode 24. Cathode 24 is maintained at a potential considerably below ground by a source of potential 34 having its positive terminal grounded. A voltage of the order of 1000 volts with respect to ground is representative of the voltage normally impressed upon cathode 24 by source 34. Focusing electrode 28, which is connected to the negative terminal of battery 32, may have a frusto-conical internal surface of revolution disposed at an angle of 671/2 degrees from its axis of symmetry. Anode 30 is maintained at a potential a few hundred volts positive with respect to the potential of cathode 24 by a connection to a tap 33 on potential source 34.

A solenoid 54 is axially positioned symmetrically about the envelope 20. An appropriate direct current is maintained in solenoid 54 by means of a potential source, such as a battery 56, so as to produce an axial magnetic field of the .order of gauss within the envelope 2li to constrain the electron stream produced by gun 22.

Proceeding along from the electron gun 22 in the direction of electron ilow, there are positioned successively about the path of the electron stream, a matching input ferrule 58 connected by an input antenna lead 60. to an input helix 62, a principal helix 63 which is,rin turn, connected by an output antenna-lead 64 to a match-l ing output ferrule 66, and a collector electrode 68 which is positioned at the end of the path so as -to collect the stream electrons. pled from input helix 62 to principal helix 63. by means of a coupling helix 65 which is disposed about the adjacent ends of input and principal helices 62 and 63.

Coupling helix 65, however, has a pitch angle negative.y

with respect to but approximately equal in magnitude to that of input and principal helices 62 and 63 to pro, vide the desired coupling and may be spaced about the4 All of the helices, which serve as the slow-wave cir-- cuit for the traveling-wave tube 10, preferably are made of a material such as tungsten or molybdenum, the principal requirement being that they retain their form,

especially with respect to the ratio of their pitches toV In accordance with the present inven-l their diameters. tion, input and coupling helices 62 and 65 are maintained at a suitable fixed potential which may be ground.,

Coupling helix 65 is connected directly to ground whereas input helix 62 is connected to ground through Alternainput antenna-lead 60 and input ferrule 58. tively, coupling helix 65 may be maintained at the same potential as that of principal helix 63, or at a cer-A tain potential between those of input helix 62 and principal helix 63. Principal helix 63 is maintained at a potential somewhat positive with respect to ground by4 a connection from output ferrule 66 to the positive terminal of a potential source 69, the negative terminal of which is grounded.

As previously mentioned, input and principal helices 62 and 63 are connected to ferrules 58 and 66 by leads 60 and 64, respectively. Leads 60 and 64 are located parallel to the electric fields excited within matching cavities 12 and 16. Matching cavity 12 has the configuration of a rectangular toroid with a concentric collar 70 disposed about and spaced from matching ferrule 5S. An opening 72 is provided in the end plate of cavity 12 facing the left end of input helix 62. Cavity 16 is similarly constructed, having a corresponding concentric collar 74 arranged about and spaced from matching (ferrule 66 and an opening 76 facing the right end of principal helix 63.

The center conductor 78 of coaxial input cable 14 ex- Electromagnetic wave energy is cou- Secam? tends through an aperture in the annular wall of cavity 12 and is connected to concentric collar 70 while the outer conductor of cable 14 is bonded to the periphery of the aperture. Likewise, the center conductor 80 of coaxial output cable 18 extends through an aperture in the annulai wall of cavity 16 and is connected to concentric collar 74 while the outer conductor of a cable 18 is bonded to the periphery of the aperture, in the same manner as before, or vice-versa. Cavities 12 and 16 are fabricated with an inner surface composed of highly conductive material and are broadly resonant so as not to limit the frequency bandwidth of operation. The configuration shown and described for the cavities 12 and 16 in the drawing, provides suitable impedance matching from input and principal helices 62 and 63 to coaxial cables 14 and 18, respectively, over a range of frequencies such as, for example, from 2000 to 4000 megacycles per second.

The stream electrons are intercepted by collector 68 at the opposite extremity of envelope 20 with respect to electron gun 22. A potential of the order of 200 volts positive with respect to ground may be applied to collector 68 in order to prevent secondary electrons, which' may be produced by the stream electrons impinging on its surface, from reaching principal helix 63 or ferrule 66. This potential isapplied by means of a connection from collector 68 to the positive terminal of a source 84', the negative terminal of whichis grounded.

In the operation of the' tube 10, an input signal to be amplified i's applied through coaxial input cable 14 to input cavity 12. The input wave in flowing along the exposed portion of conductor 78 within cavity 12 excites an electromagnetic field within that cavity. This eld induces a corresponding current in antenna-lead 60 connecti'nginput'ferrule 58 to input helix 62 to launch a travelingwave' along the'input helix 62. Interaction between the electron stream and the traveling-wave ultimately results in a net transfer of energy from the stream to the wave causing it to grow or be amplified. The wave suffers a relatively small coupling loss in the transition from input helix 62 to principal helix 63. However, the combined coupling losses will be less than the excitation loss of a single helix of the same length if an appropriate length and operating potential of input helix 62 are selected.

AtV the right end of input helix 62, wave energy is coupled from input helix 62 to principal helix 63 by coupling helix 65 thereby insuring the continuity of the wave traveling along on the helices. At the end of principal helix 63, the amplified electromagnetic wave, in flowing along output antenna-lead 64 connecting output helix 63 to output ferrule 66, excites an electric field in cavity 16. This electric field induces a corresponding output signal on center conductor 80 of coaxial cable 18.

Wave energy may be coupled from the isolated input helix 62 to the principal helix 63 in a number of ways. For example, in FIG. 2 a broken section of an evacuated envelope 120 is illustrated housing an input helix 122 which is disposed about a principal helix 124. The envlelopeV 120 is contracted at its right end whereby it is disposed contiguously about both helices 122 and 124, input helix 122 having a larger diameter than that of principal helix 124. Input helix 122 may be disposed contiguous to a portion of principal helix 124 to provide optimum electromagnetic coupling. Both of the helices 122l and 124 may then be appropriately tapered at their mutual coupling ends. It is to be noted that input helix 122 has a negative pitch with respect to principal helix 124;

The principal requirement in coupling wave energy in this manner is that all helices must propagate waves of frequencies within the operating frequency band at substantially the same velocity. The propagation velocity v1', is given approximately by where cis the velocity of light or 3 X1010 centimeters per second and P is the pitch angle of the helix, [P] being the absolute magnitude of the pitch angle. The pitch angles of helices 122 and 124 should therefore be approximately equal in absolute magnitude but may be opposite in sign.

A helix is not, of course, the only type of slow-wave structure which may be used with the tube of the present invention. Numerous other types, such as a disc-loaded waveguide, are illustrated in chapter IV in Traveling- Wave Tubes by J. R. Pierce, D. Van Nostrand and Co., New York, 1950. An input section and a principal section of a slow-wave structure there illustrated may be insulated from each other, e.g. with a dielectric ring. In that case means for maintaining the sections at different direct-current potentials and means for coupling electromagnetic energy from one section to the other may be provided in accordance with the present invention.

An electromagnetic wave coupled from external circuitry into a conductive helix will cause alternating-current velocity modulation to exist on the beam of electrons directed therealong. Part of the input signal energy thus is used to excite space charge waves on the electron stream. These waves interacting with the stream cause the total teld to be represented by three forward propagating waves Approximately one-third of th'einput signal energy will be found in each of these wavesv while only that in the growing wave is extracted as useful output energy.V An apparent loss in energy at the inp'ut of the tube is thus realized. Y

As the three excited waves travel along the helix, they interact in such away that their total voltage varies along the helixv and in addition the character ofthe totall wave as evidenced by the phase relationships between the' If the helix is severed into' voltage and current changes. two portions at some point while preservingv radio-fre'- quency continuity along the two portions of the helix, by adjusting this inter-wave interaction a wave of any desired character may be derived from the first portion to feed the second portion of the severed helix. The character o-f the wave may be modied to approximate the growing wave on the second helix portion in which case 'a larger proportion of the signal energy may go into the growing wave. This results in greater signal gain. The inter-wave interaction on the input helix can be further adjusted for optimum performance by adjusting the directcurrent voltage of the iirst helix.

In order to better understand the operation of the traveling-wave tube of the invention, it is desirable to review the gain equation of a traveling-wave tube. equation is as follows:

G=A+BCN (2) where A` is the total loss factor of the tube; B='54'..6x1v where x1 is the real part of an incremental propagation' constant 61 of the growing wave; C is a gain parameter of the tube; and N is the length of the tube 1n electronic wavelengths, i.e., the length of the tube divided by the electronic wavelength ke'=u0/f, where uo is the directcurrent electron velocity, and f is frequency.

Of the three waves propagated along the helix the growing wave is the principal contributor to the useful output power of the circuit or helix at the output end of the traveling-wave tube. v n

The loss factor A of the traveling-wave tube 10' 1s given This 5 be determined by the operating frequency range oir band fof the traveling-wave tube 10. The length of principal helix 63 will generally be determined by the radio-frequency power output or gain required of the travelingwave tubelt). Principal helix 63 will be maintained at a potential to produce `a maximum x1 in order to obtain maximum gain from the tube 16.

A2 is unaffected by changes in the length or potential of input helix 62. However', in order to determine an optimum length iand direct-current potential of input helix 62, it is necessary to plot the length of input helix 62 as a `function of If a family of curves are thus plotted for diterent values of the direct-current potential of input helix 62, then minimum loss, A40, may be plotted as `a @function of tube length. In this manner, an optimum length for the helix 62 may be found where A40 is itself a minimum. By using the optimum length, loss A4 may again be plotted as a function of the direct-current potential of the input helix 62. in this manner, an optimum direct-current potential may ybe ascertained making signal gain a maximum.

It has been determined experimentally that when two helices having an electromagnetic coupling, such as the helices 62 `and 63, having coupling helix 65, are insulated from each other in a traveling-wave tube, a substantial improvement in the gain of the tube may be obtained. The invention is thus borne out both in theory and in practice.

What is claimed is:

A traveling-wave tube amplier comprising yan electron gun including a cathode maintained at a predetermined reference potential for producing an electron stream, means for directing said stream along a predetermined path, a collector electrode disposed opposite said electron gun to intercept the stream electrons, an input helix disposed about said path adjacent said electron gun for propagating an electromagnetic wave at a predetermined velocity, said predetermined velocity being small in comparison to the velocity `of light, a principal helix electromagnetically coupled to said input helix and disposed between said input helix and said collector electrode for propagating said electromagnetic wave, means for maintaining said principal helix at a predetermined directcurrent potential producing maximum amplification of said electromagnetic wave, and means for maintaining said input helix at a predetermined direct-current potential minimizing the coupling loss to Said input helix and between both of said helices with regard to the growing wave portion of said electromagnetic wave.

References Cited in the tile of this patent UNITED STATES PATENTS 2,588,831 Hansell Mar. l1, 1952 2,660,689 Touraton et al Nov. 24, 1953 2,694,159 Pierce Nov. 9, 1954 FOREIGN PATENTS 1,053,556 France ept. 30, 1953 OTHER REFERENCES Article by P. D. Lacy, pages 132 to 135, Electronics for November 1954. 

